Participants are schedules for six evaluations in the clinic, three in each period. At each evaluation, the degree of myotonia is determined and a blood sample is taken and analyzed, after study closure, to determine Lamotrigine level. Before each evaluation participants evaluate myotonia at home by the Myotonia Behavior Scale (MBS). Furthermore, participants had to complete the validated life quality questionnaire SF-36, before first period, in the drug free period, and after the second period.